The present invention relates, in general, to minimizing the effect of mechanical vibration on the frequency of a resonant circuit element, and more particularly to a circuit element which is constructed such that the effect of mechanical vibration is minimized but still has a capability for mechanical adjustment of resonant frequency after manufacture.
Electrically resonant tuned circuits have long been used in the generation, amplification, and filtering of high frequency signals for radio, digital and analog applications. Even small changes in the resonant frequency of the circuit often have undesirable side effects, particularly if the resonator is used to determine the frequency of an oscillator. One of the principal sources of short term changes in resonant frequency stems from a microphonic effect due to mechanical vibration of the resonant circuit. Typically this microphonic effect is caused by a lack of rigidity between the circuit elements which make up the resonant circuit. While this microphonic effect can be reduced by proper design, the need for a mechanical adjustment to compensate for manufacturing variation and the physical form of the resonator limits the rigidity that can be achieved.
Resonant circuits designed to operate at frequencies over approximately 50 Mhz often take the form of a resonant transmission line segment. Fine tuning adjustment is typically accomplished by means of a capacitor coupled to the input end of the transmission line segment. This capacitance has the effect of lowering the resonant frequency by an amount which depends on the value of the capacitor. Thus adjustment of the capacitance has the effect of adjusting the resonant frequency of the resonant transmission line. The mechanical design of this adjustable capacitor combined with the requirements of mounting the capacitor and coupling it to the resonant line all serve to limit the rigidity of the structure. Another problem is the effect of the shielded enclosure for the resonator, this enclosure will couple any mechanical vibration in the structure to the resonant circuit, once again causing a microphonic effect. Clearly there is a need for a more rigid structure for resonant circuit elements such that the effects of vibration and shock are minimized.